His car-hailing company, most recently valued at $50 billion, was less than a year old at the time.

A guy named Ryan Graves saw Kalanick's tweet and responded.

"Here's a tip. email me :)," Graves wrote on Twitter. He included his email address too.Back then, Graves was a normal guy whose work experience included a database administrator position at General Electric and a stint in business development at Foursquare that he acquired by working for them for free after the company initially turned him down.

Five years later, this tweet would make a billionaire out of Graves.

Kalanick presumably followed up with Graves, who became Uber's first hire. Graves was briefly Uber's CEO before Kalanick replaced him in late 2010. Graves stayed on as Uber's head of global operations, where he remains today.

Uber's sky-high valuation makes Graves a paper billionaire today. For this reason, Graves made the cut on Business Insider's annual Silicon Valley 100 list. Kalanick, Graves, and Uber cofounder Garrett Camp also all made Forbes' list of the world's billionaires for the first time this year. Cofounders Kalanick and Camp have a larger stake in the company than Graves does, which explains their larger net worths ($5.3 billion and $5.3 billion, respectively, as opposed to Graves' $1.4 billion).

"Ryan Graves' first day was March 1st and he hit the ground running," Kalanick said in a blog post detailing Uber's origins. "From the day he got going, we spent about 15-20 hours a week working together going over product, driver on-boarding, pricing model, the whole nine. He learned the startup game fast and worked his ass off to build the Uber team and make the San Francisco launch and subsequent growth a huge success."

Uber expanded its Series E round of funding from $1.2 billion to $2.8 billion earlier this year, bringing the total amount of funding raised by Uber to an astounding $5.9 billion. The company is rumored to be raising even more money — another $1 billion — from Chinese investors, which could possibly signal an IPO soon.